This invention relates to improvements in an electric discharge gap device used, for example, in a protective device for protecting a series capacitor against an overvoltage thereacross.
In conventional electric discharge gaps with the trigger electrodes it is well known that, when a potential at the trigger electrode is opposite in polarity to that at the main electrode opposing to the trigger electrode, an electric discharge is caused across the main opposite electrodes with a small delay time. Otherwise the electric discharge occurs with a relative large delay time that is differently variable. Thus upon protecting series capacitors connected in electric power systems of alternating current against overvoltages, the use of such conventional electric discharge gaps has been fatally disadvantages in that in each half cycle of the system voltage in which the trigger electrode thereof is applied with a potential having the same polarity as that at the main electrode opposing to the trigger electrode, the particular system fault may cause the dielectric breakdown of the series capacitor due to an abnormal increase in voltage thereacross within a time interval between the occurrence of that fault and the initiation of an electric discharge across the main electrode of the associated discharge gap. In order to avoid that fatal disadvantage, one could use a pair of conventional electric discharge gaps with the trigger electrode connected in parallel relationship across a series capacitor. However this measure is disadvantageous in that the resulting protective device becomes large-sized and objectionable in view of the maintenance and economy involved.